a) Field of the Invention
This invention relates to a Karman's vortex flow meter in which a difficulty is eliminated by provision of O-rings or damping members that the resonance of its amplifier's casing with an external vibration adversely affect a detecting section concerning Karman's vortex to lower the measurement accuracy.
b) Related Art
A conventional Karman's vortex flow meter will be described with reference to FIG. 5.
As shown in FIG. 5, a vortex generator 2 is inserted into a pipe line 1 in which fluid under test flows, in such a manner that the vortex generator 2 is extended diametrically across the pipe line 1. The vortex generator 2 comprises a columnar member provided on the upstream side (on the left of FIG. 5) and another columnar member provided on the downstream side (on the right of FIG. 5) (hereinafter referred to as "an upstream columnar member" and "a downstream columnar member", respectively, when applicable). In order to sealingly hold the fluid in the pipe line 1 and to prevent vibration at the junction of the vortex generator 2 and the pipe line 1, both end portions of the vortex generator 2 are engaged with the wall of the pipe line 1 through O-rings 5 and 6. A detecting bar 3 comprises: diskshaped diaphragm; a lower bar portion extended axially inwardly (downwardly in FIG. 5) from the diaphragm; and an upper bar portion which is threaded and extended axially outwardly (upwardly in FIG. 5) from the diaphragm (all having no reference numerals). The diaphragm is in contact with the upper end face of the vortex generator 2 through an O-ring 24. The lower end portion of the bar body, which is smaller in diameter than the remaining portion, is inserted through a sealing and vibration-proofing O-ring 4 into a hole formed in the downstream columnar member of the vortex generator 2. A piezo-electric element 7, an insulating plate 8, and a disk spring 9 are mounted on the threaded upper bar portion of the detecting bar 3 in the state order, and then fixedly secured with a balance weight 10 engaged with the threaded upper bar portion. The piezo-electric element 7 detects even a slight displacement of the small-diameter lower end portion of the detecting bar 3 as a displacement of the diaphragm, to output an electrical signal. The balance weight 10 serves as a nut being threadably engaged with the threaded upper bar portion of the detecting bar to secure the piezo-electric element 7, the insulating plate 8 and the disk spring 9 as was described above. In addition, the balance weight 10 has a function of eliminating the effects of vibrations, such as the vibration of the pipe line 1, other than those due to Karman's vortex.
On the other hand, a supporting stand 22 comprises a flange, and a hollow cylinder extended from the flange. The flange is fixedly mounted on the pipe line 1 with bolts 11 to fixedly secure the vortex generator 2 to the pipe line 1. The hollow cylinder incorporates a detecting section concerning Karman's vortex which includes the diaphragm of the vortex generator and the piezo-electric element 7. A casing assembly for a signal amplifier (hereinafter referred to as "an amplifier casing assembly", when applicable) is fixedly fitted on the hollow cylinder of the supporting stand 22. The amplifier casing assembly comprises: a casing 13; and two covers 14 which are threadably secured through O-rings 16 to the casing 13 from both sides. The amplifier casing assembly holds a printed circuit board inside, which includes a detecting circuit for obtaining a flow rate from the output of the piezo-electric element 7, and other circuits. The lower end portion of the casing 13 is fitted through one O-ring 15 on the upper end portion of the hollow cylinder of the supporting stand 22, and secured with a pair of right and left bolts 17. The O-ring 15 is used to sealingly isolate the interior containing the detecting section and the detecting circuit from the exterior.
The above-described conventional Karman's vortex flow meter is disadvantageous in that it is liable to be affected by external vibrations. That is, when the amplifier casing assembly resonates with an external vibration, the resonance is transmitted through the supporting stand 22 to the detecting section including the piezo-electric element 7, thus affecting the output of the latter 7. Hence, in the case where the pipe line vibrates greatly to oscillate the Karman's vortex flow meter, and the flow rate is small, the measurement of flow rate is adversely affected by the vibration, thus being low in accuracy.